Approximately 180,000 women are diagnosed with breast cancer each year in the United States. Most of these women are cured of their disease by surgery and local radiotherapy. However, nearly 60,000 women go on to develop metastatic breast cancer each year, and 45,000 of these patients eventually die from their malignancies. While metastatic breast cancer is rarely curable, it is treatable with modern pharmaceuticals that prolong patient survival and reduce the morbidity associated with metastatic lesions. Foremost among these therapies are hormonal manipulations that include selective estrogen receptor modifiers (SERMs). SERMs are small ligands of the estrogen receptor that are capable of inducing a wide variety of conformational changes in the receptor and thereby eliciting a variety of distinct biological profiles. SERMs not only affect the growth of breast cancer tissue but also influence other physiological processes. The most widely used SERM in breast cancer is tamoxifen, which is a partial estrogen receptor agonist/antagonist that produces objective responses in approximately 50% of the patients. Unfortunately, 100% of patients who take tamoxifen eventually relapse with tamoxifen-resistant tumors. Approximately 50% of the patients that fail tamoxifen treatment will respond to a subsequent hormonal manipulation therapy such as castration, aromatase inhibitors, or other SERMs. The second line therapies for hormonal manipulation therapy of metastatic breast cancer represent a substantial unmet need because no single agent has become the treatment of choice for patients who fail tamoxifen therapy. The ideal agent would be a medication that induces regression of metastatic breast cancer lesions in women who have previously responded to tamixofen therapy.
SERMs modulate the proliferation of uterine tissue, skeletal bone density, and cardiovascular health, including plasma cholesterol levels. In general, estrogen stimulates breast and endometrial tissue proliferation, enhances bone density, and lowers plasma cholesterol. Many SERMs are bifunctional in that they antagonize some of these functions while stimulating others. For example, tamoxifen, which is a partial agonist/antagonist at the estrogen receptor inhibits estrogen-induced breast cancer cell proliferation but stimulates endometrial tissue growth and prevents bone loss. Estrogens are an important class of steroidal hormones that stimulate the development and maintenance of fundamental sexual characteristics in humans. In the past, estrogens have been found useful in the treatment of certain medical conditions and diseases. For example estradiol, a steroid hormone produced by the ovary, is useful in the treatment of osteoporosis, cardiovascular disease, premenstrual syndrome, vasomotor symptoms associated with menopause, atrophic vaginitis, Kraurosis vulvae, female hypogonadism, primary ovarian failure, excessive hair growth and prostatic cancer.
Hormone replacement therapy (HRT) with estrogen has been determined to be a clinically effective treatment for osteoporosis in post-menopausal women. However, less than 15% of eligible women are currently prescribed HRT despite clinical trials that have demonstrated a 50% reduction in hip fractures and a 30% reduction in cardiovascular disease. Non-compliance arises from patient and physician concerns over the two fold increased risk of endometrial cancer observed with HRT employing estrogen alone as well as the association between estrogen therapy and breast cancer. Although unproven in the clinic, this suspected risk for breast cancer has led to HRT being contraindicated in a significant percentage of post-menopausal women. Co-therapy with progestins has been shown to protect the uterus against cancer while maintaining the osteoprotective effects of the estrogen, however the progestin introduces other side effects such as withdrawal bleeding, breast pain and mood swings.
In light of the more serious side effects associated with estrogen therapy, including myocardial infarction, thromboembolism, cerebrovascular disease, and endometrial carcinoma, a significant amount of research has been carried out to identify effective nonsteroidal estrogen and antiestrogenic compounds. In general, such compounds may be characterized as both estrogenic and antiestrogenic because, while they all bind to the estrogen receptor, they may induce an estrogenic or antiestrogenic effect depending upon the location of the receptor. In the past, it has been postulated that the binding of various nonsteroidal estrogen and antiestrogenic compounds to the estrogen receptor was due to the presence of a common pharmacophore (shown below in Scheme A), which was recurrent in the chemical structures of these compounds.

This pharmacophore later became the structural backbone around which nonsteroidal estrogen and antiestrogenic compounds were constructed. Its presence in the constructs of various compounds such as hexestrol, tamoxifen, chroman, triphenylethylene, DES, clomiphene, centchroman, nafoxidene, trioxifene, toremifene, zindoxifene, raloxifene, droloxifene, DABP, TAT-59 and other structurally related compounds has become accepted in the art as the molecular key to estrogen receptor binding specificity.
Estrogen has also been shown to function as a mitogen in estrogen-receptor (ER) positive breast cancer cells. Thus, treatment regiments which include antiestrogens, synthetic compounds which oppose the actions of estrogen have been effective clinically in halting or delaying the progression of the disease (Jordan and Murphy, Endocrine Reviews 11:578–610 1990); Parker, Breast Cancer Res. Treat. 26:131–137 (1993)). The availability of these synthetic ER modulators and subsequent dissection of their mechanism(s) of action have provided useful insights into ER action.
The human estrogen receptor (ER) is a member of the nuclear receptor superfamily of transcription factors (Evans, Science 240:889–895 (1988)). In the absence of hormone, it resides in the nucleus of target cells in a transcriptionally inactive state. Upon binding ligand, ER undergoes a conformational change initiating a cascade of events leading ultimately to its association with specific regulatory regions within target genes (O'Malley et al., Hormone Research 47:1–26 (1991)). The ensuing effect on transcription is influenced by the cell and promoter context of the DNA-bound receptor (Tora et al. Cell 59:471–487 (1989) (Tasset et al., Cell 62:1177–1181 (1990); McDonnell et all Mol. Endocrinol. 9:659–669 (1995); Tzukerman et al. Mol. Endocrinol. 8:21–30 (1994)). It is in this manner that the physiological ER-agonist, estradiol, exerts its biological activity in the reproductive, skeletal and cardiovascular systems (Clark and Peck, Female Sex Steroids:Receptors and Function (eds) Monographs Springer-Verlag, New York (1979); Chow et al., J. Clin. Invest. 89:74–78 (1992); Eaker et al. Circulation 88:1999–2009 (1993)).
One of the most studied compounds in this regard is tamoxifen (TAM), (Z)1,2-diphenyl-1-[4-[2-(dimethylamino) ethoxy]phenyl]-1-butene, (Jordan and Murphy, Endocrine Reviews 11:578–610 (1990)), which is a triphenylethylene derivative. Tamoxifen functions as an antagonist in most ER-positive tumors of the breast and ovum, but displays a paradoxical agonist activity in bone and the cardiovascular system and partial agonist activity in the uterus (Kedar et al. Lancet 343:1318–1321 (1994); Love et al., New Engl. J. Med. 326:852–856 (1992); Love et al., Ann. Intern. Med. 115:860–864 (1991)). Thus, the agonist/antagonist activity of the ER-tamoxifen complex is influenced by cell context. This important observation is in apparent contradiction to longstanding models that hold that ER only exists in the cell in an active or an inactive state (Clark and Peck, Female Sex Steroids:Receptors and Functions (eds) Monographs on Endocrinology, Springer-Verlag, New York (1979)). It indicates instead that different ligands acting through the same receptor can manifest different biologies in different cells. Definition of the mechanism of this selectivity is likely to advance the understanding of processes such as tamoxifen resistance, observed in most ER-containing breast cancers, where abnormalities in ER-signaling are implicated (Tonetti and Jordan, Anti-Cancer Drugs 6:498–507 (1995)). Tamoxifen, as well as a structurally similar compound known as raloxifene have been developed for the treatment and/or prevention of osteoporosis, cardiovascular disease and breast cancer in addition to the treatment and/or prevention of a variety of other disease states. Both compounds have been shown to exhibit an osteoprotective effect on bone mineral density combined with a positive effect on plasma cholesterol levels and a greatly reduced incidence of breast and uterine cancer. Unfortunately, tamoxifen and raloxifene both have unacceptable levels of life-threatening side effects such as endometrial cancer and hepatocellular carcinoma.
The likely mechanism for the cell selective agonist/antagonist activity of tamoxifen has been determined using an in vitro approach (Tora et al., Cell 59:477487 (1989); Tasset et al., Cell 62:1177–1187 (1990); McDonnell et al., Mol. Endocrinol. 9:659–669 (1995); Tzukerman et al., Mol. Endocrinol. 8:21–30 (1994)). Importantly, it has been shown that tamoxifen induces a conformational change within ER which is distinct from that induced by estradiol (McDonnell et al., Mol. Endocrinol. 9:659–669 (1995); (Beekman et al., Molecular Endocrinology 7:1266–1274 (1993)). Furthermore, determination of the sequences within ER required for transcriptional activity indicate how these specific ligand-receptor complexes are differentially recognized by the cellular transcriptional machinery. Specifically, it has been shown that ER contains two activation domains, AF-1 (Activation Function-1) and AF-2, which permit its interaction with the transcription apparatus. The relative contribution of these AFs to overall ER efficacy differs from cell to cell (Tora et al., Cell 59:477–487 (1989); McDonnell et al., Mol. Endocrinol. 9@65-9-669 (1995); Tzukerman et al., Mol. Endocrinol. 8:21–30 (1994)). Estradiol was determined to function as both an AF-1 and an AF-2 agonist, in that it exhibited maximal activity regardless of which AF was dominant in a given cellular environment. Tamoxifen, on the other hand, functions as an AF-2 antagonist, inhibiting ER activity in cells where AF-2 is required or is the dominant activator (Tora et al., Cell 59:477–487 (1989); McDonnell et al., Mol. Endocrinol. 9:659–669 (1995); Tzukerman et al., Mol. Endocrinol. 8:21–30 (1994)). Conversely, tamoxifen functions as an agonist when AF-1 alone is required (McDonnell et al., Mol. Endocrinol. 9:659–669 (1995); Tzukerman et al., Mol. Endocrinol. 8:21–30 (1994)). Subsequently, based on their relative AF-1/AF-2 activity, four mechanistically distinct groups of ER-modulators were defined; full agonists (i.e. estradiol), two distinct classes of partial agonists, represented by tamoxifen and raloxifene, and the pure antagonists, of which ICI182,780 is a representative member (McDonnell et al., Mol. Endocrinol. 9:659–669 (1995); Tzukerman et al., Mol. Endocrinol. 8:21–30 (1994)). These results provide a mechanistic explanation for the observed differences in the biological activities of some ER-modulators and indicate that the mechanism by which ER operates in different tissues is not identical.
Interestingly, the agonist activity exhibited by ER-modulators, such as estrogen and tamoxifen, in these in vitro systems reflects their activity in the reproductive tracts of whole animals. This correlation does not extend to bone, however, where estradiol, tamoxifen and raloxifene, which display different degrees of AF-1/AF-2 agonist activity, all effectively protect against bone loss in the ovariectomized rat model. Thus, with the exception of the steroidal pure antiestrogens (ie, ICI182,780), all known classes of ER modulators appear to protect against bone loss in humans and relevant animal models, while they display different degrees of estrogenic activity in other tissues (Chow et al., J. Clin. Invest. 89:74–78 (1992); Love et al., New Engl. J. Med. 326:852–856 (1992); Draper et al., Biochemical Markers of Bone and Lipid Metabolism in Healthy Postmenopausal Women. In C. Christiansen and B. Biis (eds) Proceedings 1993. Fourth International Symposium on Osteoporosis and Consensus Development Conference, Handelstrykkeriet, Aalborg; Wagner et al., Proc. Natl. Acad. Sci. USA 93:8739–8744 (1996); Black et al., J. Clin. Invest 93:63–69 (1994)).
A series of non-steroidal compounds that retain beneficial characteristics such as osteoprotective activity while minimizing any undesirable side effects would be most advantageous. While it is presently accepted that the pharmacophore backbone mentioned above is responsible for estrogen receptor binding specificity, it has now been discovered that certain novel estrogen binding ligands can be constructed as described herein which incorporate particular moieties onto such pharmacophore-based compounds, thereby maximizing beneficial characteristics such as osteoprotective function while minimizing undesirable characteristics such as an increased risk of cancer.
The present invention provides selective estrogen receptor modulators, which retain beneficial characteristics while minimizing undesirable side effects such as increased risk of cancer.